Carboxylic acids are recognized to be highly toxic to marine life. In the refining industry, they are commonly referred to as organic acids and are generally a mixture of saturated higher fatty acids. They are also often incorrectly grouped together as naphthenic acids or simply "nap acids". For refineries processing "high neutralization number" crudes such as California and some Gulf crudes, even very low levels of organic acids can result in a refinery's inability to meet aquatic compliance requirements on effluent waters. Rainbow trout fry are particularly sensitive to such compounds.
Carboxylic acids are classified by the chemical structure R--COOH. In this acid form, they are fully hydrocarbon soluble and generally water insoluble. Only those organic acids with a carbon count of five or less exhibit any water solubility. However, an important characteristic of organic acids is that alkali metal salts of these compounds are readily soluble in water and insoluble in hydrocarbon media. That is, carboxylic acids and their alkali metal salts show opposite solubility behavior.
Certain crude oils thus contain organic acids which are typically present as salts such as sodium or calcium naphthenates. During the raw crude desalting process, these water-soluble (alkali metal) salts are extracted into the resulting tail water. A number of refineries have dealt with such undesirables by removing them through an extractive process wherein the acid-laden stream is first acidified with H.sub.2 SO.sub.4 to a pH of less than 2.0 in order to convert the salts into the oil-soluble acid form. While still in the acid phase, the aqueous stream is then extracted with a "sponge" oil (diesel or similar distillate) thereby removing the organic acids from the water stream. The pH of the water is then adjusted back up to neutrality for subsequent handling in the wastewater system.
While reasonably effective, this process has several drawbacks. First, the low pH precludes the use of carbon steel and dictates the use of more expensive, stainless steel metallurgies. Second, because of crude impurities and emulsion stabilizers, the acid-laden sponge oil is typically rather wet (up to 40% water) and can release naphthenate-laden water upon separation when mixed with higher pH waters in tankage. This can result in the re-introduction of organic acid species into the effluent water system. Lastly, the acidification step can result in the potential release of H.sub.2 S vapors that could impose significant potential health hazards. The solutions to be treated may already possess some level of H.sub.2 S and the use of sulfuric acid would only increase this ancillary treatment problem.
It would be desirable if a method could be discovered which would remove organic acids from aqueous solutions without these attendant disadvantages.